Modern jet engines often operate under elevated heat loads. Under such extreme conditions, varnish, or carbonaceous deposits, may be formed from the oxygenation and polymerization reactions of hydrocarbons in jet fuel, aviation turbine fuel, or kerosene fuel.
Jet fuel generally includes hydrocarbons as the main component. As shown in FIG. 1, under an elevated heat load in the fuel system, the hydrocarbon molecule R may be activated to form a radical R., after removal of H. by another radical or oxidant initiator. Subsequently, radical R. may be subjected to attack by oxygen gas or dissolved oxygen (O2) to form a peroxy radical, ROO., and may further form a hydroperoxide ROOH by taking a hydrogen radical from another hydrocarbon molecule R′, thereby initiating a chain reaction. Hydroperoxides are the primary product in this “autoxidation” process at moderate temperatures (<250° F.) but they further decompose at higher temperatures to propagate fuel autoxidation mechanisms. Varnish products occur when the fuel radicals R. react and polymerize, thereby forming insoluble large molecular weight species which precipitate to form a condensed varnish film or deposit. The radical or undercoordinated species on the outside of this film can react with incoming radicals to further build the film.
Varnish typically causes clogging in aircraft fuel system components, such as actuators, valves, filters, and bearings, and significantly degrades the performance of the fuel system. Generally, the removal of dissolved oxygen from the fuel using a fuel stabilization unit (FSU) is necessary for suppressing varnish formation in jet engine systems (U.S. Pat. Nos. 6,315,815; 6,719,492; 7,615,104; 8,177,884; 7,413,818, 7,744,827). Accordingly, various FSUs to remove dissolved oxygen have been developed. For example, a FSU membrane has been reported to effectively remove the dissolved oxygen (U.S. Pat. Nos. 7,465,335; 8,070,859). Alternatively, the FSU may comprise an electrochemical or physical device to remove dissolved oxygen (U.S. Pat. Nos. 8,147,600; 8,177,884).
It is anticipated that, in the near future, advanced jet turbine engines will require substantial increases in fuel heat loads in order to operate with high efficiency, which will accelerate varnish formation. As such, the removal of dissolved oxygen alone may not be sufficient and advanced FSU may further require removal of chemical precursors which promote varnish formation.
Moreover, other fuel contaminants may include metals and heteroatomic organic ligands, for example, copper (Cu) and naphthenic acid. Among other contaminants, the heteroatomic ligands or contaminants mostly originate from the crude oil source feed from which the jet fuel is refined. However, since the heteroatomic contaminants have substantially close boiling fractions to the kerosene-type species in the jet fuel, the undesired heteroatomic contaminants remain in jet fuel even after the refining process. For example, naphthenic acids, cycloalkanes with carboxylic acid functional groups and alkyl side groups, are some of the most deleterious fuel contaminants for varnish formation. The heteroatomic species can react with metals, such as Cu, zinc (Zn), and iron (Fe), to form metal-ligand complexes which are soluble in the jet fuel, like the dissolution of Cu by naphthenic acids to form Cu(II) di-naphthenate (FIG. 2). Such metal-ligand complexes may act as catalysts and/or co-reactants that decompose saturated hydrocarbon components in the jet fuel, facilitate fuel peroxide formation, and hydroperoxide breakdown leading to radical formation. As described above, the radicals promote fuel autoxidation and polymerization, leading to varnish formation. Particularly, at elevated heat loads, e.g. at temperatures of about 250° F. and higher, such autoxidation and polymerization reactions may be greatly accelerated. Therefore, the removal of the heteroatomic contaminants in the fuel or fuel system may be critical for the performance and maintenance of advanced aircraft engines and fuel systems.
As such, there is a need for developing a method or a device to remove fuel contaminants in jet fuel that are precursors for varnish formation.
The description provided above as a related art of the present invention is just for helping understanding the background of the present invention and should not be construed as being included in the related art known by those skilled in the art.